I managed to get the router into recovery mode and reflash it with the ASUS firmware. I had to try unplugging & re-plugging it 8-10 times before it would go into recovery.
After getting it up and working with the stock image, it still had the settings I'd made before trying to flash Tomato. Seemed a little weird, so I did a factory reset and reconfigured it.
Then I tried flashing the build you mentioned but it didn't seem to work. After flashing with the ASUS utility, the router won't successfully reboot. I waited maybe 10 minutes before attempting to power cycles the router, but it didn't help. The switch port light comes on and my laptop is detecting that the link is up so it's a different failure mode than before, but the router doesn't seem to boot. Some other lights (power and WLAN, I think) are coming on as well.
Does Tomato have a reset method like DD-WRT's 30/30/30? Does it use the restore button or the WPS button on the RT-N12?
I tried flashing DD-WRT via recovery as well to see if it would work, but ran into the same symptoms. Something weird seems to be going on. Could the bad flash have somehow corrupted some of the flash in a way that is not being reset by recovery attempts?